Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!sharkey!itivax!umich!don
From: don@zippy.eecs.umich.edu (Don Winsor)
Newsgroups: sci.electronics
Subject: Re: How can I recognize true ground?
Message-ID: <178@zip.eecs.umich.edu>
Date: 11 Jul 89 20:02:40 GMT
References: <18425@mimsy.UUCP> <1547@unccvax.UUCP> <3283@kitty.UUCP> <5651@stiatl.UUCP>
Reply-To: don@zip.eecs.umich.edu.UUCP (Don Winsor)
Organization: University of Michigan EECS Dept., Ann Arbor, MI
Lines: 35

In article <5651@stiatl.UUCP> john@stiatl.UUCP (John DeArmond) writes:
> ... Our poster has the problem of only 2 wires - a hot and a
> neutral.  But no safety ground.  In these instances, the only
> real alternative is to run a safety ground to the outlet.
> In some cases, the neutral is connected to the outlet
> box/ "ground pin".  This is worse than nothing at all.

I've run into a poor wiring job in an older house belonging to
friends where I found a multitude of sins.  One of them was that
some grounded receptacles were wired with old two conductor cable
and they just had the neutral and ground pins tied together at
the receptacles.  In the interests of safety I suggested they
pull modern NM cable to these receptacles, which didn't seem too
hard to do, since there was easy access down through the wall and
into the (unfinished) basement.  My friend didn't think this was
necessary, and I couldn't come up with a good, convincing argument
that this was a bad idea.  His line of reasoning was that the
ground and neutral wires all went to the same bus bar on the
main breaker panel, so running another conductor offered no
advantage.  Can anyone offer a good, clear, convincing argument
to refute this?  About the best I could think of was if the neutral
wire got broken somewhere between the breaker panel and the
receptacle, and you then plugged a low resistance device with a
grounded case into the outlet (say an electric drill with a three
prong cord in a metal case), there could be a significant leakage
path from hot, through the motor windings, out the drill's neutral
wire, across the neutral-ground tie in the receptacle, and back up
the drill's ground wire to the drill case.  Thus, we now have a
hot drill.  Is this the reasoning?  If not, can someone give a
better explanation as to why "this is worse than nothing at all"?

                Don Winsor
                Electrical Engineering and Computer Science
                University of Michigan, Ann Arbor, Michigan
                don@dip.eecs.umich.edu